Lithium secondary batteries (inclusive of lithium ion secondary batteries) are widely utilized in portable electronic equipment, personal computers, and the like. While miniaturization and weight reduction are required for the lithium secondary batteries, increasing the energy density is an important problem to be solved.
There are several methods for increasing the energy density of a lithium secondary battery, and among them, increasing the operating voltage of a battery is effective. A lithium secondary battery using lithium cobaltate or lithium manganate as a positive electrode active material has an average operating voltage of 3.6 to 3.8 V (4 V class) versus a metal lithium reference. This is because the operating voltage is defined by the oxidation-reduction reaction of cobalt ions or manganese ions (Co3+⇄Co4+ or Mn3+⇄Mn4+).
On the other hand, a spinel compound in which a part of manganese in lithium manganate is replaced by nickel or the like, specifically LiNi0.5Mn1.5O4 or the like, shows a potential plateau in a region of 4.5 V or more. Therefore, by using the spinel compound of this type as a positive electrode active material, 5 V class operating voltage can be achieved. In a positive electrode using the spinel compound, manganese is present in the tetravalent state, and the operating voltage of the battery is defined by the oxidation-reduction of Ni2+⇄Ni4+ instead of the oxidation-reduction of Mn3+⇄Mn4+.
LiNi0.5Mn1.5O4 has a capacity of 130 mAh/g or more and an average operating voltage of 4.6 V or more versus metal lithium, and has smaller lithium absorbing capacity than LiCoO2 but has higher energy density than LiCoO2. For such a reason, LiNi0.5Mn1.5O4 is promising as a positive electrode material.
On the other hand, improvement of the life characteristics is a problem that has always been required in lithium batteries. Various causes are said to be the reason of deterioration of battery life. For example, the decomposition reaction of the electrolytic solution at a contact portion of a positive electrode active material with the electrolyte solution has been pointed out.
In order to suppress the decomposition on the positive electrode active material, there are several techniques for treating the surface of the positive electrode active material. For example, there is a proposal to cover the surface of an active material with a metal oxide, as disclosed in Patent Document 1 and Patent Document 2.
As the reports relating to crystallinity of 5V class positive electrodes, Non-Patent Document 1 and Non-Patent Document 2 are known. Non-Patent Document 1 has shown characteristics such as differences in interfacial resistance depending on the difference in crystal structure due to presence or absence of Ni ordering. Non-Patent Document 2 has shown that the crystal structure of P4332 is obtained even when Mn was replaced with Ti in LiNi0.5Mn1.5O4. Thus, it has been reported that the reactivity with an electrolyte solution at the interface is different depending on the control of the crystal structure, or that it is possible to control the crystallinity of an active material by conditions.